Flammable materials have been studied in normal gravity and microgravity for many years. Photography plays a major role in the study of the combustion process giving a permanent visual record that can be analyzed. When these studies are extended to manned spacecraft, safety becomes a primary concern. The need for a high-pressure, flame-resistant, shatter-resistant window permitting photographic recording of combustion experiments in manned spacecraft prompted the development of a method for bonding Lexan and sapphire. Materials that resist shattering (e.g., Lexan) are not compatible with combustion experiments; the material loses strength at combustion temperatures. Sapphire is compatible with combustion temperatures in oxygen-enriched atmospheres but is subject to shattering. Combining the two materials results in a shatter-resistant, flame-resistant window. Combustion in microgravity produces a low-visibility flame; however, flame propagation and flame characteristics are readily visible as long as there is no deterioration of the image. Since an air gap between the Lexan and the sapphire would reduce transmission, a method was developed for bonding these unlike materials to minimize light loss

Richardson, William R.

Walker, Ernie D.

English

NASA Technical Reports Server

NASA Technical Memorandum 100188 
. 
Bonding Lexan and Sapphire to 
Form High-pressure, Flame- 
Resistant Window 
William R. Richardson and Ernie D. Walker 
Lewis Research Center 
Cleveland, Ohio 
( N A S A - T H -  100188) BCWDING L B l A E  A I D  S A P P H I B E  ~ ~ - 2 a m 0  
I C  POEW hXGB-EBESSURB,  PLAI!E-bESIST41T 
P I Y D O Y  ( N A S A )  8 F A v a i l :  P ~ X S  nC A02/#lP 
A01 CSCL 13H Unclas  
G3/35 0097554 
Prepared for the 
129th Technical Conference and Equipment Exhibit 
sponsored by the Society of Motion Picture and Television Engineers 
Los Angeles, California, October 31-November 4, 1987 . 
https://ntrs.nasa.gov/search.jsp?R=19870019447 2020-03-20T10:14:48+00:00Z
BONDING LEXAN AND SAPPHIRE TO FORM HIGH-PRESSURE, FLAME-RESISTANT WINDOW 
Q, 
rD 
I 
W 
Wi l l i am R. Richardson and Ern ie  D. Walker 
Nat ional  Aeronaut ics and Space Admin i s t ra t i on  
Lewi s Research Center 
Cleveland, Ohio 441 35-31 91 
SUMMARY 
Flammable ma te r ia l s  have been stud ied under l abo ra to ry  condi t i o n s  i n  nor- 
mal g r a v i t y  and m ic rog rav i t y  for many years.  Photography p lays  a major r o l e  
i n  the  study o f  the combustion process as i t  g ives a permanent v i s u a l  reco rd  
t h a t  can be analyzed. When these s tud ies are  extended i n t o  manned spacecraf t ,  
personnel sa fe ty  becomes a pr imary concern. The need for a high-pressure, 
f lame- res is tan t ,  sha t te r - res i s tan t  window p e r m i t t i n g  photographic reco rd ing  o f  
combustion experiments i n  manned spacecraf t  prompted the  development o f  a 
method fo r  bonding Lexan and sapphire. Ma te r ia l s  t h a t  r e s i s t  s h a t t e r i n g  (e.g., 
Lexan) a re  not compatible w i t h  combustion experiments as the  ma te r ia l  loses 
s t reng th  a t  combustion temperatures. 
temperatures i n  oxygen-enriched atmospheres b u t  i s sub jec t  t o  sha t te r i ng .  Com- 
b i n i n g  the  two mate r ia l s  r e s u l t s  i n  a sha t te r - res i s tan t ,  f lame- res is tan t  win- 
dow. Combustion i n  m ic rog rav i t y  produces a l o w - v i s i b i l i t y  f lame; however, 
f lame propagat ion and flame c h a r a c t e r i s t i c s  a re  r e a d i l y  v i s i b l e  as long as 
there  i s  no d e t e r i o r a t i o n  of the  image. 
and the  sapphire would reduce transmission, a method was developed for  bonding 
these u n l i k e  ma te r ia l s  t o  minimize l i g h t  loss .  
Sapphire i s  compat ib ie w i t h  combustion 
Since an a i r  gap between the  Lexan 
TRANSMISSION MEASUREMENTS 
The Lexan-sapphire windows were developed for use i n  reco rd ing  combustion 
i n  m ic rog rav i t y ,  where the absence of convect ion r e s u l t s  i n  a b a r e l y  v i s i b l e  
flame. The development of  a bonding procedure was necessary because o f  the  
l a r g e  s i z e  and i r r e g u l a r  shape of  the windows. According to measurements made 
wi th  a Macbeth t ransmission densitometer i n  the  v i s i b l e  l i g h t  spectrum (-400 to  
700 pm), the  window mate r ia l  absorbs some of the  l i g h t .  The o u t e r  window sur- 
. face, a 1/2- inch- th ick Lexan b lock,  2-314 inches by 5-114 inches, was measured 
f i r s t .  The t y p i c a l  r e s u l t ,  as shown i n  t a b l e  I, was a t ransmiss ion d e n s i t y  
( l i g h t  loss) o f  0.06 t o  0.09, where 0.30 equals one f-stop. 
r i a l  dens i t y  measured c o n s i s t e n t l y  0.02. When the two mate r ia l s  were sand- 
wiched b u t  n o t  bonded, the expected dens i ty  of 0.08 to 0.11 a c t u a l l y  was 0.13 
t o  0.16. 
The sapphire mate- 
I n  an e f f o r t  to  improve the transmission, we declded to  bond the  two mate- 
r i a l s  w i t h  an u l t r a v i o l e t - c u r a b l e  o p t i c a l  adhesive and thus e l i m i n a t e  the  a i r  
space. Norland o p t i c a l  adhesive #61 was tes ted  f i r s t  as i t  met MIL-A-3920 and 
was approved for  a l l  government contracts  spec i fy ing  such adhesives. The use 
of  an approved bonding agent would s imp l i f y  approval for use i n  crew quar te rs  
aboard the  space s h u t t l e .  
The use of #61 was d iscont inued a f te r  several  windows separated dur ing ,  or 
w i t h i n  24 hours of, cur ing .  
i n c l u d i n g  clamping the windows w i th  spr ing clamps, e l a s t i c s ,  or w i r e  for  the  
Several d i f f e ren t  ho ld ing  arrangements were t r i e d  
cur ing  process. 
sources, and exposure t imes were t r i e d  i n  an e f f o r t  to  ge t  a good c l e a r  bond 
w i thout  any separat ion.  
o p t i c a l  adhesive #68. 
f l e x i b l e  than #61. 
I n  a d d i t i o n ,  changes i n  c lean ing  agents, bonding agents, l i g h t  
The successful method invo lved  the  use o f  Nor land 
The adhesive a l s o  meets  MIL-A-3920 and i s  s l i g h t l y  more 
PROCEDURE 
Cleaning 
The f i r s t  step i n  assembling a window was c lean ing  the  surfaces o f  the two 
mate r ia l s .  Bonded a lcoho l  and then ammonium hydrox ide app l i ed  wi th  a c o t t o n  
pad (Webri 1 Wipes) worked e f f e c t i v e l y .  Several appl i c a t i o n s  o f  the  s o l u t i o n s  
were  necessary, and care had to  be taken i n  scrubbing the  Lexan w i t h  the  c o t t o n  
pad as the  surface cou ld  be scratched. The surfaces were then d r i e d  w i t h  a 
c lean c o t t o n  pad and inspected for  defects .  White co t ton  gloves were worn dur- 
i n g  c lean ing  and assembly. 
A f t e r  cleaning, each window was v i s u a l l y  inspected and any imper fec t ions  
i n  e i t h e r  the Lexan or the sapphire were documented. 
made w i t h  a b lack background and i l l u m i n a t e d  w i t h  a s t rong,  d i r e c t i o n a l  s ide  
l i g h t  ( f i g .  1 ) .  Th is  i nspec t i on  s t a t i o n  a l lowed observa t ion  o f  very  small 
defects .  
An inspec t i on  s t a t i o n  was 
Each surface was then cleaned again w i t h  e t h y l  a lcoho l  (200 p roo f )  and 
ammonium hydroxide, app l ied  w i t h  a co t ton  pad. 
repeated several  t i m e s  u n t i l  no v i s i b l e  res idue remained. 
cleaned w i t h  ion ized a i r  to  remove dus t  and s t a t i c  e l e c t r i c  charges. 
The c lean ing  process was 
Surfaces were then 
Assembl y 
The f l a t t e s t  s ide  of  the Lexan p iece was p laced face  up on a c lean,  f l a t  
surface. Approximately 15 drops o f  Norland o p t i c a l  adhesive #68 were app l i ed  
to  the sur face i n  a p a t t e r n  t h a t  had been found t o  spread evenly ( f i g .  2). 
Trapped a i r  bubbles were then removed from the  cement, and the  cement was 
a1 lowed t o  s tab i  1 i z e  i n  order  t o  reduce sur face depressions t h a t  t r a p  ' a i r .  
The sapphire p l a t e  was then lowered on to  the sur face of  the  cement, pushing 
trapped a i r  toward the edges ahead o f  the contac t  f r o n t  ( f i g .  3 ) .  The sap- 
p h i r e  p l a t e  spread the cement, which formed an ova l  spot between the  surfaces 
( f i g .  4 ) .  
Small bubbles o f t e n  remained trapped between the surfaces. 
could be forced out by p lac ing  a weight between the  center  of the sample and 
the a i r  bubble and moving the weight toward the  nearest  edge t o  follow the 
m i g r a t i n g  bubble ( f i g .  5). 
spread of cement between the two surfaces and t o  remove a l l  the  a i r .  
weight of the sapphire ma te r ia l  f o rced  excess cement from between the two sur- 
faces. 
from the  edges, the sandwich was placed i n t o  the  ho ld ing  f i x t u r e .  The f i x t u r e  
prevented the sapphire from s l i d i n g  around on the cement before i t  became tacky 
( f i g .  6). The sandwiched m a t e r i a l s  were then exposed to long-wave u l t r a v i o l e t  
l i g h t  fo r  10 minutes t o  s e t  the sample. 
Usual l y  they  
I t  genera l l y  took about 112 hour t o  ge t  an even 
When the cement stopped seeping o u t  and the excess had been removed 
The 
The l i g h t  source was a Pen-Ray model 
2 
SCT-1 p laced approximately 2 inches from the sapphire surface. 
cleaned, inspected, and then exposed to  the  lamp for  6 hours to  cure. 
exposure a r e f l e c t o r  was p laced over the lamp and the  window to  concentrate 
the  l i g h t  and to  p r o t e c t  personnel ( f i g .  7 ) .  
The window was 
Dur ing 
Sal vagi  ng Sapphi r e  Mater1 a1 s 
Early procedures r e s u l t e d  i n  fa i l u res ,  making i t  necessary to  separate 
some windows and salvage the sapphire. 
soak t h e  window i n  a s o l u t i o n  of bonded a lcohol  and ammonium hydroxide. 
s o l u t i o n  d isso lved the  Lexan mater ia l  i n  approximately 24 hours. 
p o r t i o n  of  the window was l e f t  w i t h  a hard res idue t h a t  requ i red  extens ive 
c l  eani ng by hand. 
The f i r s t  sa lvaging method used was to 
The sapphire 
The 
A more successful method of separat ion was to  s lowly  r a i s e  the  tempera- 
The window was placed i n  a shal low t u r e  o f  the sample i n  a l abo ra to ry  oven. 
conta iner  w i t h  the sapphire s ide  up. The conta iner  was placed i n  the  cool  
weii and t h e  temperature w a s  raised to approximaieiy 230 'F over  i 5  to  20 min- 
u tes .  
This  u s u a l l y  l e f t  a l a y e r  of adhesive t h a t  could be peeled o f f  i n  sheets. Any 
remain ing res idue cou ld  be q u i c k l y  cleaned i n  so lvent .  
The heat re leased the adhesive and the sapphire could be l i f t e d  o f f .  
TESTIflG 
The completed windows ( f i g .  8)  were i n s t a l l e d  i n  a pressure vessel 
( f i g .  9) and h y d r o s t a t i c a l l y  t es ted  to 60 ps ig .  The pressure vessel was then 
pressur ized  w i t h  2 atmospheres o f  helium and placed i n  a vacuum chamber evacu- 
a ted  to 10-5 torr.  A leak check was conducted. 
CONCLUSIONS 
Upon complet ion of the  t e s t s  t h e  windows showed no s ign  of  separat ion and 
remained o p t i c a l l y  c lea r .  A total  o f  e i g h t  windows were f a b r i c a t e d  and t e s t e d  
us lng  t h e  procedure o u t l i n e d .  
concluded t h a t  t h i s  procedure i s  acceptable for bonding l a rge  windows f a b r i -  
cated from u n l i k e  ma te r ia l s .  
There were no f a i l u r e s .  From the  r e s u l t s  we 
TABLE I. - TRANSMISSION DENSITY OF WINDOW MATERIALS 
[Average o f  e i g h t  windows.] 
Ma t e r i  a1 
Sapphi r e  
Lexan 0 .06  t o  0.09 
Sapphi re-Lexan (unbonded) 0.13 t o  0 .16  
0.08 t o  0.11 
3 
I 
'Q 
( A )  TOP VIEW. (B) S IDE VIEW. 
FIGURE 1. - INSPECTION STATION. 
C E E N T  POOL 
FIGURE 2. - ADHESIVE APPLICATION PATTERN. 
(A) TOP VIEW. 
AIR BUBBLES P\ ,- CONTACT / FRONT 
LEXAN L CEMENT SAPPHIRE 
BLOCK POOL 
(B)  SIDE VIEW. 
FIGURE 3. - ASSEMBLY OF LEXAN-SAPPHIRE WINDOW. 
4 
QRlGffdAL PAGE IS 
OF POOR QUALITY 
7 LEXAN BLOCK 
I 
\ !  
I 
L5 
FIGURE 4. - SPREADING OF CEMENT. 
c-87-3706 
FIGURE 6 .  - HOLDING FIXTURE. 
4 
FIGURE 5 .  - BUBBLE MIGRATION. 
FIGURE 7. - CURING STATION. 
5 
FIGURE 8. - COMPLETED LEXAN-SAPPHIRE WINDOW. 
FIGURE 9.  - PRESSURE VESSEL. 
6 
National Aeronaulrs and 
I .  Report No. 
NASA TM-100188 
Report Documentation Page 
2. Government Accesslo ' n No. 
17. Key Words (Suggested by Author@)) 
Bonding unlike optical materials 
7. Author(s) 
William R. Richardson and Ernie D .  Walker 
18. Distribution Statement 
Unclassified - Unlimited 
Subject Category 35 
___ 
9. Performing Organization Name and Address 
National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 44135-3191 
National Aeronautics and Space Administration 
Washington, D.C. 20546-0001 
2. Sponsoring Agency Name and Address 
19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No of pages 
Unclassified Unclassified 7 
3. Recipient's Catalog No. 
22. Price' 
A02 
5. Repolt Date 
6. Performing Organization Code 
8. Performing Organization Report No. 
E-3769 
10. Work Unit No. 
694-03-03 
11. Contract or Grant No. 
13. Type of Report and Period Covered 
Technical Memorandum 
14. Sponsoring Agency Code 
5. Supplementary Notes 
Prepared for the 129th Technical Conference and Equipment Exhibit sponsored by 
the Society of Motion Picture and Television Engineers, Los Angeles, 
California, October 31 - November 4, 1987. 
16. Abstract 
Flammable materials have been studied under laboratory conditions in normal 
gravity and microgravity for many years. 
study of the combustion process as it gives a permanent visual record that can 
be analyzed. When these studies are extended into manned spacecraft, person- 
nel safety becomes a primary concern. 
resistant, shatter-resistant window permitting photographic recording of com- 
bustion experiments in manned spacecraft prompted the development o f  a method 
for bonding Lexan and sapphire. 
are not compatible with combustion experiments as the material loses strength 
at combustion temperatures. 
tures in oxygen-enriched atmospheres but is subject to shattering. Combining 
the two materials results in a shatter-resistant, flame-resistant window. Com- 
bustion in microgravity produces a low-visibility flame; however, flame propa- 
gation and flame characteristics are readily visible as long as there is no 
deterioration of the image. 
phire would reduce transmission, a method was developed for bonding these 
unlike materials to minimize light loss. 
Photography plays a major role in the 
The need for a high-pressure, flame- 
Materials that resist shattering (e.g., Lexan) 
Sapphire is compatible with combustion tempera- 
Since an air gap between the Lexan and the sap- 
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Bonding Lexan and sapphire to form high-pressure, flame-resistant window

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